So long as it's going from smaller to bigger in the direction of flow, you can't go too far wrong.

I've got a similar problem with 4V Cleveland heads where I bought exhaust gaskets and had some plates laser cut to them in order to make some manifolds.
I made a few mistakes on the way of course because the gaskets don't fit the ports properly (didn't check first) and I didn't make allowance for wall thickness.
That's why properly built engines are so expensive, lots of hours to get it right.

When dealing with "stock " parts, that's the way the step goes.
With aftermarket parts, you're usually dealing with random low quality bits which never fit properly.........just a generalisation.

You can take pretty much any engine from Japan (best quality) and the intake ports in the head are always bigger than the intake manifold ports. On the exhaust side, the holes in the exhaust manifold are always bigger than the ports in the head. The step always goes the correct way and it's usually even all the way around.
The step in the correct direction doesn't do a lot to airflow but a step in the wrong direction makes a mess of it.

OP you are way overthinking this. In your particular application, as long as you don't have a crapload of gasket protruding into the runner you're good to go. FWIW I just slapped on a 7104 intake with 1206 gasket on AFR 210 heads on a 400sbc. It's a super responsive engine that pulls hard to 6000rpm.

OP you are way overthinking this. In your particular application, as long as you don't have a crapload of gasket protruding into the runner you're good to go. FWIW I just slapped on a 7104 intake with 1206 gasket on AFR 210 heads on a 400sbc. It's a super responsive engine that pulls hard to 6000rpm.

The cam has very much to say here. Are you buildin max effort?

Look at the picture, I posted. It's worse than that. I have a metal "wall" form the heads' intake port protruding into the runner (if the gasket is a fair indicator).

When tuning exhaust lengths, you essentially stop exhaust tuning and set the "end of the pipe" virtually by using exhaust resonator boxes- a large increase in volume along the middle of the exhaust length, appears to the first part of the exhaust as the outside air and the pressure wave hits the end of the tube at the boundary with the terminator box and terminates/ resonates back down the exhaust from there.

I'm buying a long-runner, tuned-length intake because of it's sole benefit- a tuned long length maximizes filling at lower RPM and produces significant increases in cylinder filling / pressure / torque. If the air is coming down the runners and then all of a sudden opens up to a large void how does that not dramatically impact the intake tuning, causing the runner to behave as if it were much shorter? -Now I'm ruining my intake tuning and velocity and I'm losing far more torque than I'm ever going to gain in HP...

I'm starting to think having the intake ported smaller than the gasket (same width because everything fits there, but closer to the Felpro 1205 height, rather than the 1206 height) might be the way to go. -If I have the intake ported to the gasket size and then have to open up the heads' intake ports even further that's just opening a bigger void and slowing the airflow down even further...

OP you are way overthinking this. In your particular application, as long as you don't have a crapload of gasket protruding into the runner you're good to go. FWIW I just slapped on a 7104 intake with 1206 gasket on AFR 210 heads on a 400sbc. It's a super responsive engine that pulls hard to 6000rpm.

The cam has very much to say here. Are you buildin max effort?

Look at the picture, I posted. It's worse than that. I have a metal "wall" form the heads' intake port protruding into the runner (if the gasket is a fair indicator).

Now place the gasket on the intake. As long as the intake runner from the manifold is smaller than the heads runner, you have no problems. You shouldn't even have a problem if the intake manifold runners were slightly bigger than the heads. Port mismatch is very common and some believe in port matching and blending runners more than others. I believe in it but on a build like yours the benefits are probably not even noticed.

The head runners are very short compared to the intake manifold. Especially with your First tpi manifold.

anti-reversion like alot of stuff in that thread often has a much bigger mismatch, if you look at a stepped header it offers some antireversion capabilities but not like some other things out there as it is often only a 1/16" radial step.

my thoughts are it depends on how close the areas are to what the engine needs.

for example if you have a given head with given port size and bolt on a smaller intake manifold the step up in area creates a small loss depending on the ratio of the mismatch areas. if you ran a bigger manifold you get a slightly higher loss (look up step up and step down pipe losses tables its not a huge difference for the most part to small mismatches) but your manifold area is increased. So if the smaller manifold was just way too small the bigger manifold might feed the engine better despite the higher loss at the interface (i.e. net effect is positive) . the extra turbulence might help to ? a stepped intake might act like a crude taper to